The present invention relates to the flying height of disk drive transducer heads over the surface of magnetic storage disks. In particular, the present invention relates to the detection of transient high fly write conditions in a computer disk drive.
Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks, divided into sectors. Information is written to and read from a disk by a transducer head, mounted on an actuator arm capable of moving the transducer head radially over the disk. Accordingly, the movement of the actuator arm allows the transducer head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors on the disk. The transducer head may include separate or integrated read and write heads.
In a typical computer disk drive, the transducer head generally rides above the surface of the disk on a cushion of air that is created by the movement of the disk under the head. The distance of the head from the surface of the magnetic disk is known as the flying height of the transducer head. It is important to maintain the flying height of the transducer head within a desired range. For example, if the transducer head flies at too low a height, it is more likely to come into contact with the magnetic disk causing a loss of stored data. It is also important to ensure that the transducer head does not fly at too great a height. Where the transducer head is consistently at too great a distance from the magnetic disk, the transducer head is said to be in a xe2x80x9chigh flyingxe2x80x9d condition. A xe2x80x9chigh fly write eventxe2x80x9d occurs when the transducer head suddenly is at too great a height from the disk to reliably perform write operations.
Data written to a magnetic storage disk for storage while a transducer head is experiencing a high fly write condition may be lost. This is because the strength of the magnetic field generated by the write head decreases exponentially with distance. Accordingly, where the transducer is at too great a distance from the surface of the magnetic disk (e.g., during a high fly write event), the magnetic field produced may not be strong enough to induce the desired magnetic transitions in the storage disk. Therefore, it is important to detect a high fly write event in a computer disk drive, so that writing to the disk may be delayed until the transducer head has returned to the desired flying height above the surface of the disk.
Previous methods of detecting high fly write events have included monitoring the amplitude of a signal produced in the read head when the read head passes over an automatic gain control (AGC) field on the disk. In general, automatic gain control fields are written to computer disk drive storage disks during manufacturing. AGC fields are usually located in xe2x80x9cservo sectorsxe2x80x9d or xe2x80x9chard sectors,xe2x80x9d which are areas extending radially across the disk that contain servo positioning information. In general, the automatic gain control fields provide a reference magnetic field strength, so that the gain added to read signals by the hard disk drive""s channel can be appropriately adjusted.
During the manufacture of a hard disk drive, servo sector information is written to the disk. During this xe2x80x9ctrack writingxe2x80x9d procedure, transducer heads having a write head that is about ⅔ the width of a data track are used. Because the AGC fields are the width of an entire data track, each AGC field must be written by at least three passes of the write head. This xe2x80x9cstitchingxe2x80x9d together of the AGC fields causes the AGC fields to be vulnerable to manufacturing defects that result in AGC fields with unevenly spaced magnetic transitions, that in turn result in magnetic fields of uneven strength. These manufacturing defects result in the production of signals in the read head that vary in amplitude. Although these manufacturing defects do not affect the functioning of the AGC fields in connection with their primary gain control function, it prevents high fly event detection systems dependent upon the amplitude of the AGC field from operating with high sensitivity. In addition to defects within individual AGC fields, defects resulting in undesired variations in the magnetic strength of the AGC fields may arise between adjoining AGC fields, where the fields are aligned radially across all or a portion of the disk.
Sensitivity is important in detecting high fly write events, because such events are transient in nature. For example, a high fly write event may occur when a transducer head passes over a particle on the surface of the disk. The particle may cause a perturbation in the boundary layer of air supporting the transducer head, causing the head to fly at greater than a desired distance from the disk surface. In a typical high fly event, the transducer head flies at too great a height for one or two sectors of the disk. Therefore, it can be appreciated that detection of a high fly write event preferably is made on the basis of information collected over a single sector of the disk. Furthermore, because the change in height is small, and because the high fly write event may last for extremely short periods of time, it is important that the high fly write detection mechanism be extremely sensitive, so that high fly write events can be detected even at the beginning or end of such an event.
Another method of detecting a high fly write event is disclosed by U.S. Pat. No. 5,909,330 to Carlson et al., assigned to the assignee of the present invention. According to Carlson, whether the flying height of a read/write head above a disk is within an acceptable range can be determined by the resolution of a signal read by the read head. This method is different from the present invention in that it relies upon the decrease in the number of detected signal peaks as the flying height of the read head increases.
For the above stated reasons, it would be advantageous to provide a method and apparatus for quickly and reliably sensing a high fly write event in a computer disk drive. In addition, it would be advantageous to provide a method and apparatus for detecting high fly write events that did not adversely affect the performance of the disk drive. Furthermore, it would be advantageous to provide such a method and apparatus that can be implemented at an acceptable cost and that is reliable in operation.
In accordance with the present invention, a method and apparatus for detecting the flying height of a disk drive transducer head over the surface of a magnetic storage disk is provided. According to the present invention, the strength or amplitude of a signal derived from servo sector position bursts written to the magnetic disk is monitored to determine the height of the transducer head over the magnetic disk. The present invention allows high fly events to be detected, even where the transducer head is in a high fly condition for only a very brief period of time.
According to an embodiment of the present invention, the average amplitude of a signal derived from a servo sector position burst or group of servo sector position bursts is determined. The amplitude of signals derived from the passage of a servo sector position burst or group of bursts is then compared to the average value. If the observed amplitude is less than the average amplitude, a write fault error may be triggered. Typically, a write fault error prevents attempts at writing information to the magnetic disk for a period of time. According to one embodiment of the present invention, the transducer head is prohibited from performing write operations for a time that is equivalent to the approximate amount of time required for at least two servo sectors on the magnetic disk to pass beneath the transducer head.
According to another embodiment of the present invention, the amplitude of a signal derived from a servo sector position burst or a group of bursts is compared to a value or values derived from, but not exactly equal to, a calculated average amplitude. For instance, a high fly write event may be triggered only if the observed amplitude of a signal derived from a servo sector position burst or a group of bursts is less than an average signal amplitude by at least a predetermined amount.
According to yet another embodiment of the present invention, the observed amplitude of a signal derived from a servo sector position burst or a group of bursts may be compared to a range of values derived from an average signal amplitude. According to this embodiment, the present invention may be utilized to detect both high fly events and occurrences when the transducer head flies at too low a height.
According to a further embodiment of the present invention, the observed strength or amplitude of a signal derived from a servo sector position burst or a group of bursts may be compared to a calibrated value. The calibrated value may be obtained during calibration procedures performed during the manufacture or closely following the manufacture of the disk drive. Such a calibrated value may be substituted for the average value described in connection with any of the embodiments of the present invention described herein.
As yet a further embodiment of the present invention, an average or calibrated threshold value may be determined according to the zones present on the magnetic disk. Accordingly, the amplitude of an observed signal derived from a servo sector position burst or a group of bursts located in a particular zone may be compared to an average or calibrated threshold amplitude for that particular zone.
Based on the foregoing summary, a number of salient features of the present invention are readily discerned. A computer disk drive having the capability of detecting high fly write events, and thereby helping to ensure the integrity of data written to a disk, is provided. In particular, a method and apparatus for detecting high fly write events with great sensitivity is provided.